Contamination of subsurface soil and ground water by petroleum products, including gasoline and fuel oils, is a recurring problem, particularly by leakage from underground storage tanks. In addition, spills and improper disposal of petroleum products cause many contaminated sites.
Among the diverse remediation technologies which have been developed to treat contaminated subsurface soil and ground water are the techniques of air sparging and bioremediation.
In general, air sparging involves the injection of high volumes of pressurized air into ground water through injection wells. The air flow through the ground water causes volatile contaminants to volatilize and migrate through the ground water and the vadose zone. To avoid contamination of the environment the ensuing vapors are directed to extraction wells and withdrawn from the wells under vacuum. The output of the withdrawal well is connected to a series of scrubbers, condensers and neutralizers which remove contaminants from the air stream.
Bioremediation relies on the ubiquitous microorganisms found in soil and ground water and in vadose zones which are capable of biodegrading the contaminants. Aerobic microorganisms convert hydrocarbon fuels into carbon dioxide and water. This method relies on oxygen availability and nutrients in the soil to support the microorganisms. Without enhancing the oxygen availability it is expensive and difficult to sustain a successful rate of biodegradation.
Combining the techniques of air sparging and bioremediation provides an integrated system which is capable of enhancing the bioremediation of the various zones of contamination underground by increased rates of biodegradation. Air contains 80% nitrogen and 20% oxygen and is not an efficient source of oxygen and requires high volumes of air to provide sufficient oxygen. This integrated system still causes the undesirable migration and volatilization of volatile contaminants through the vadose zone. Release of such vapors above ground must be controlled to meet air quality standards and this requires venting and extraction of the vapors. In addition high volume gas flow causes drying out of the zones and channeling which leads to progressively more leakage and escape of vapors and reduced efficiency of biodegradation. Drying out of the soil due to the air flow can render it biologically inactive. Channeling caused by high volume air flow reduces the efficiency of oxygen transfer to the aqueous phase and hence to the microorganisms. Maintaining a moist environment and avoiding channeling over the term of the operation are important for efficient biodegradation.
A pump-and-treat method of in situ bioremediation has been reported (Carter, S. R. and Clark, J. E.; Oxygen-Enhanced In Situ Bioremediation In A Sand And Gravel Aquifer, pp 551-558 of In Situ Aeration: Air Sparging, Bioventing, and Related Remediation Process, Ed. Robert E. Hinchee, Ross N. Miller and Paul C. Johnson. Battelle Press 1995). The remediation system included ground water recovery and oxygenation of the ground water in a packed-tower air stripper to remove volatiles and increase dissolved oxygen levels. The oxygenated water was injected back to the subsurface through horizontal injection galleries.
U.S. Pat. Nos. 5,221,159; 5,277,518 and 5,472,294 disclose methods and apparatuses for in situ removal of contaminants from soil or from ground water, or both by combination sparging and bioremediation techniques which volatilize and biodegrade the contaminants.
U.S. Pat. No. 5,277,518 discloses a method and apparatus for in situ removal of contaminants from soil or from ground water, or both by (col. 3, lines 46-54) establishing at least one injection well into the ground water; injecting oxygen-containing gas into the injection well to volatilize and biodegrade contaminants in the ground water and soil; and permitting upward migration of the volatilized and biodegraded contaminants without one or more venting conduits. In other words, no venting wells, venting laterals, or equivalent equipment is used. A sparger in the injection well introduces pressurized air into the aquifer so as to stimulate circulation of the air and mixture of the air and water. This remediation unit requires the use of air injection wells to volatilize contaminants which then migrate through the vadose zone and are vented to the atmosphere. Some of the contaminants are biodegraded by microorganisms in the soil and ground water. The air flow rates must be controlled so that the air quality standards are not exceeded by the fluids vented by the remediation unit.
U.S. Pat. No. 5,472,294 discloses a method and apparatus for in situ removal of contaminants from soil or from ground water, or both in a similar manner as disclosed in U.S. Pat. No. 5,277,518. In addition an apparatus (FIG. 11) is provided which allows the addition of supplemental oxygen-containing gas, including oxygen gas, at high pressure and low volume through a feed tube attached to the sparger. The feed tube provides the oxygen-containing gas to an expander to more widely disperse the gas into the contaminated zone.
It is an object of this invention to provide a method for the efficient removal of biodegradable contaminants from soil or ground water, or both without causing migration or volatilization of the contaminants.
It is an object of this invention to provide a method for the efficient removal of biodegradable contaminants from soil or ground water, or both without releasing the contaminants into the atmosphere.
It is an object of this invention to provide a method for the efficient removal of biodegradable contaminants from soil or ground water, or both by enhancing the rate of biodegradation of the contaminants by native microorganisms in the soil or ground water.
It is an object of this invention to provide a method for the efficient removal of biodegradable contaminants from soil or ground water, or both by ensuring complete dissolution of injected oxygen into the ground water.